(strings foo.adt | grep -i "(wmo|m2|mdx)" | while read file; do strings $file | grep -i blp; done) | sort -u
As per his screenshot, it seems the issue is with the various wmo/m2 files, not necessarily the adt tilesets.
That line will result in all blps used in all objects in your art file, so it will pretty much be what he requested.
